This invention relates to membranes useful for removal of ions imparting hardness to water.
A variety of treatments are employed for the removal of hardness factors from water. These hardness factors are principally calcium or magnesium salts of sulfate or bicarbonate. Lime softening is widely used, but requires large quantities of lime and certain practical limitations. Ion exchange is widely used where the cost of the ion exchange resin and regeneration of the resin are not prohibitive. Electrodialysis has been employed, but is relatively costly.
Reverse osmosis membranes have been employed for water softening. Cellulose diacetate membranes and polyamide membranes have been used commercially for this purpose. However, these prior art membranes in general have required relatively high operating pressures and have been viewed as energy intensive.
U.S. Pat. No. 3,551,331 describes the treatment of certain polyamide membranes with acid. This treatment, performed on melt-processed linear aliphatic polyamide membranes of extremely low water permeability, produced higher water permeabilities by a combination of solvent induces crystallization and solvent extraction of low molecular weight non-crystalline fractions. The resulting membranes still required high operating pressures to achieve commercially feasible water permeation rates.
U.S. Pat. Nos. 3,853,755 and 3,866,066 describe the use of hydrolyzable tannins to modify the rejection of reverse osmosis membranes. However, the colloid in general reduces flux through a membrane.
A membrane which affords a combination of high flux, adequate rejection of hardness ions and very low operating pressures has long been sought. More specifically, a membrane which has a flux of at least about 10, preferably at least about 15 gallons per square foot per day (gfd) and a magnesium sulfate rejection of at least about 85 percent at a transmembrane pressure in the range from about 50 to about 100 pounds per square inch (psi), preferably from about 50 to about 70 psi, and temperature of 25.degree. C. using feed water containing 0.2 weight percent magnesium sulfate would be desirable.